U.S. Pat. No. 3,710,695 by Messrs. Charles P. Miller, Allen R. Miller and David J. Miller discloses a curb and gutter machine wherein the main frame is supported upon two pairs of crawler tractors, each pair being steerable in unison on vertical steering axes. The front and rear tractors on one side (outboard) of the main frame have individually controlled vertically adjustable support means which control the grade of machine and the slip form tool as it progresses along the path of travel. The front and rear tractors on the other side (inboard) of the main frame are connected to the ends of a walking beam that extends horizontally along that side of the main frame. A single vertically adjustable support means is affixed centrally of this other side of the main frame and is pivotally connected on a transverse axis to the center of the walking beam for slope control.
The machine described in this patent is quite versatile in that not only does it provide finite and automatic control of the grade and slope of the finished product, it can be easily maneuvered into position along a grade line and the steering capabilities allow the machine to follow the grade line exactly as it progresses along the path of travel. This is because both the front and rear pairs of tractors steer in unison and independently so that the machine can make sharp turns as well as maintain itself and the frame tangential to outside curves and in a chordal relationship to inside curves.
Since the machine has a large degree of vertical adjustment capability at each vertical support, the dual steering linkage for each pair of tractors must extend and retract in unison as the machine negotiates transverse offsets in the grade as shown in FIG. 3 of the patent. To accomplish this, referring to FIG. 4 of the patent, the front pair of linkages 168 and 170 is provided with the compensating cylinders 190 and 192 which are cross-connected by means of the hydraulic lines 198 and 200 so that the exhaust of one becomes the intake of the other. The rear pair of steering linkages 172 and 174 has the compensating cylinders 208 and 210 that are cross-connected in the same manner by the hydraulic lines 212 and 214.
Thus, the respective pairs of rods 194 and 196 and the rods 215 of these four compensating cylinders are allowed or forced to extend and retract in unison to lengthen the steering arms or linkages to accommodate the greater or lesser distances from the other tractors to which these linkages are affixed.
This steering system works well and is quite durable. However, it is to be observed that each of these compensating cylinders is so connected that the side of one cylinder which includes the piston rod (for present purposes called the replacement chamber) is connected to the side of the other cylinder which does not have a rod therein (the displacement chamber). One would assume that since the actual volumetric displacement on each side of the piston in each ram or cylinder is different by the amount of hydraulic oil displaced by the rods, the displacement ahead or behind each piston as it moves would be less than or greater than the volume of the other portion of the dual system to which it is connected and the rams would lock up. It actual practice, however, the system contains sufficient air, about 5% by volume, which is compressible, to off-set this difference. However, the result is that the full extension and retraction potential of each compensating ram is not obtained and unnecessary limitations are placed on this function of the machine.
It is a feature of the machine of said patent to provide extreme versatility as far as operability is concerned over rough grade and only that portion of the grade upon which the curb and gutter is to be laid need be graded with any exactitude. This greatly reduces the costs of construction and allows the machine to be used under conditions that are not possible with other construction machines that are designed to lay curb and gutter or other such extruded concrete structures. Regardless of this extreme versatility the machine is often used under such adverse grade conditions by zealous or incompentent operators that its grade compensating functions are exceeded and mechanical failures may result. This invention is intended to overcome this limitation in the steering and leveling functions of machines having at least a pair of tractors which must steer and simultaneously control grade under extreme conditions so often imposed by operators of such machinery in the construction field.